It is often desirable to compensate for the detrimental effects of the atmosphere upon light transmitted therethrough. Phase misalignments of a light beam transmitted through the atmosphere occur because of spatial and temporal variations in the density and chemical composition of the atmosphere. These distortions can be considered to occur in a series of wavefronts along the beam's propagation direction.
More particularly, variations in the density and chemical composition of the atmosphere cause corresponding variations in the index of refraction of the atmosphere. These variations in the index of refraction cause undesirable distortion of the phase of a light beam's wavefronts. That is, the index of refraction variations lead to local speed of light variations, causing differences in the optical path length experienced by different portions of a wavefront.
Such distortion has detrimental effects upon light beams used in various applications, including optical communications, imaging, and weaponry. In optical communications, atmospheric distortion can substantially limit bandwidth. In imaging, atmospheric distortion can undesirably reduce the resolution of telescopes and cameras. In directed energy weaponry, atmospheric distortion can limit the amount of power delivered within a given area upon a target.
Wavefront correction can be used to compensate for atmospheric distortion. Contemporary systems for wavefront correction, particularly in high power applications, commonly use mechanical devices such as deformable mirrors. Such mechanical devices are useful because the large surface areas within their active correction regions readily accommodate high laser powers. However, such mechanical devices tend to be undesirably bulky and heavy. Thus, their use has a significantly detrimental impact upon launch and flight costs for space-based systems.
The use of spatial light modulators for wavefront correction is known. For example, a phase-only dual-frequency liquid crystal spatial light modulator can vary the phase relationships of portions of a wavefront on a pixel-by-pixel basis. In this manner, phase misalignments caused by differences in the index of refraction for the optical paths traveled by different portions of a light beam can be mitigated.
The amount of wavefront correction needed for each portion of a light beam tends to vary generally continuously. Thus, each pixel of the spatial light modulator needs to react generally continuously and in real time, so as to provide the desired correction.
The light beam should be monitored with sufficient accuracy as to facilitate such real-time control of the spatial light modulator. That is, the relative phase relationships of the different portions of the light beam need to be measured and control signals then provided to the spatial light modulator so as to facilitate accurate varying of the phase delays introduced thereby on a pixel-by-pixel basis. However, variations in the temperature of a spatial light modulator can vary it response, thus inhibiting its ability to respond accurately to control signals. For example, variations in the temperature of a dual-frequency liquid crystal spatial light modulator can undesirably cause corresponding variations in the crossover frequency thereof.
As a result, there is a need for a method and system for accurately determining the amount of wavefront phase correction needed and then quickly and precisely providing wavefront phase correction, particularly wherein the detrimental effects of temperature variations of the phase correction device are mitigated.